This invention relates to controllable electronic-circuit oscillators and more particularly to calibration of controllable oscillators in phase-locked loops.
Phase locked loops (PLLs) are well known, and are useful in many types of circuits, including but not limited to radio circuitry. In digital communication systems, for example in mobile telephone communications operating under the GSM or DCS systems, PLLs may be employed to effect continuous phase modulation (CPM) of a carrier signal.
PLLs are often used to generate signals having frequencies that are integer divisions of reference frequencies, and fractional-N PLLs have been devised to achieve more flexible division factors. A fractional-N PLL includes a variable-modulus divider, rather than an integer divider. For example, performing three successive divisions by 20 followed by one division by 21 results in an average division factor of (3.20+21)/4=20.25.
Sigma-delta-controlled fractional-N PLLs are often used in radio systems to generate spurious-free local oscillator frequencies and to allow faster frequency jumps. By controlling the divider ratio with a sigma-delta modulator, modulation with a constant envelope can be generated. By using these properties of the fractional-N PLL, compact radio architectures for constant-envelope systems (e.g., GSM, DCS) can be developed. This also means that the complete radio can be integrated in the same application-specific integrated circuit (ASIC).
Sigma-delta-controlled fractional-N PLL modulators are described in, for example, U.S. Pat. No. 6,049,255 to H. Hagberg et al. Other uses of PLLs include phase detectors, such as those described in, for example, U.S. Pat. No. 6,605,935 to M. Nilsson et al.
U.S. Pat. No. 6,111,471 to Bonneau et al. describes an apparatus for setting the free-running frequency of a voltage-controlled oscillator (VCO) to a reference frequency. The apparatus includes a means for setting the frequency range of the VCO, a first counting means for counting to a first value at the VCO rate, a second counting means for counting to a second value at the reference frequency rate. The second counting means produces a reference count value when the first counting means reaches the first value. A state machine is responsive to the two counting means and selects a VCO frequency range such that the VCO-free running frequency is closest to the reference frequency based on a comparison of the duration of a decrementing reference count from the second counting means with the counting duration of the second counting means.
Other devices for setting the free-running frequency of a VCO are described in U.S. Pat. No. 5,746,712 to Branstad et al.; U.S. Pat. No. 5,694,087 to Ferraiolo et al.; U.S. Pat. No. 5,406,228 to Hoang; U.S. Pat. No. 5,394,444 to Silvey et al.; U.S. Pat. No. 5,220,295 to Glover et al.; and U.S. Pat. No. 5,160,900 to Visuri; European Patent Publications EP 1170869 by Paananen; and EP 1168627 by Hirano et al.; and Japanese Patent Abstract JP 58/107729 by Ogino.
In some radio designs, only one mandatory trimming, or adjustment, is required, and that is trimming of the frequency of a VCO in the PLL. In general, the trimming process proceeds as follows: set a start value of the control voltage; measure the oscillator's output frequency; check whether the measured frequency is within the target range; if the measured frequency is not within the target range, step the voltage value and repeat the loop; and if the measured frequency is within the target range, store the voltage value in the flash memory of the device.
To ensure accuracy, this trimming can be done only at the final test of the complete end product, e.g., a mobile phone. Such late testing is undesirable, and is further undesirable because it consumes significant time in production and requires sophisticated radio-frequency test instruments and software. It also affects the design of the device to the extent that some kind of storage must be provided in the device for the trim value. If this kind of trimming could be eliminated, it would significantly reduce production costs.